In order to prevent or reduce skull and brain injuries many activities requires helmets. Most helmets consist of a hard outer shell, often made of a plastic or a composite material, and an energy absorbing layer called a liner. Nowadays, a protective helmet has to be designed so as to satisfy certain legal requirements which relate to inter alia the maximum acceleration that may occur in the center of gravity of the brain at a specified load. Typically, tests are performed, in which what is known as a dummy skull equipped with a helmet is subjected to a radial blow towards the head. This has resulted in modern helmets having good energy-absorption capacity in the case of blows radially against the skull while the energy absorption for other load directions is not as optimal.
In the case of a radial impact the head will be accelerated in a translational motion resulting in a linear acceleration. The translational acceleration can result in fractures of the skull and/or pressure or abrasion injuries of the brain tissue. However, according to injury statistics, pure radial impacts are rare.
On the other hand, a pure tangential hit that results in a pure angular acceleration to the head are rare, too.
The most common type of impact is oblique impact that is a combination of a radial and a tangential force acting at the same time to the head, causing for example concussion of the brain. The oblique impact results in both translational acceleration and rotational acceleration of the brain. Rotational acceleration causes the brain to rotate within the skull creating injuries on bodily elements connecting the brain to the skull and also to the brain itself.
Examples of rotational injuries are on the one hand subdural haematomas, SDH, bleeding as a consequence of blood vessels rupturing, and on the other hand diffuse axonal injuries, DAI, which can be summarized as nerve fibers being over stretched as a consequence of high shear deformations in the brain tissue. Depending on the characteristics of the rotational force, such as the duration, amplitude and rate of increase, either SDH or DAI occur, or a combination of these is suffered. Generally speaking, SDH occur in the case of short duration and great amplitude, while DAI occur in the case of longer and more widespread acceleration loads. It is important that these phenomena are taken into account so as to make it possible to provide good protection for the skull and brain.
The head has natural protective systems that try to dampen these forces using the scalp, the hard skull and the cerebrospinal fluid beneath it. During an impact, the scalp and the cerebrospinal fluid acts as rotational shock absorber by both compressing and sliding over the skull. Most helmets used today provide no protection against rotational injury.
Important features of for example bicycle, equestrian and ski helmets are that they are well ventilated and have an aerodynamic shape. Modern bicycle helmets are usually of the type in-mould shell manufactured by incorporating a thin, rigid shell during the molding process. This technology allows more complex shapes than hard shell helmets and also the creation of larger vents.